1. Field of the Invention
This invention is related to metal sulfide powders useful as precursors for optical ceramics and catalysts. More particularly, this invention is related to a low temperature synthesis of metal sulfide powders by reaction between an organometallic compound and hydrogen sulfide.
2. Description of the Prior Art
Metal sulfide powders are useful as precursors for optical ceramics used in sensor windows and domes on aircraft, satellites, and missiles. Precursors having high purity and small. uniform particles are necessary to provide optical ceramics having excellent thermal, mechanical and optical properties. Additionally, metal sulfide powders are useful as catalysts because they contain small, uniform particles having a large surface area.
Conventional methods of synthesizing metal sulfide powders require high temperatures and inorganic starting materials. High temperatures promote particle growth which detracts from the optical and thermomechanical properties of the ceramic. Inorganic starting materials contain undesirable impurities which can degrade the performance of the optical ceramic or catalyst.
Organometallic compounds are known to be useful in the preparation of some metal chalcogenides. For example, the gas phase reaction between an organometallic compound such as diethyl zinc with hydrogen sulfide, hydrogen selenide or dimethyltellurium has.been reported. The reaction is run at temperatures of 750.degree. C. or greater and has been used to prepare the sulfides, selenides and tellurides of zinc and cadmium.
The liquid phase reaction of diethyl zinc with hydrogen sulfide has been reported. This room temperature reaction has been used to prepare zinc sulfide containing ligands such as 2,2'-bipyridine, 1,10-phenanthroline, and pyridine. The procedure involves an H.sub.2 S purge of an anhydrous ethereal solution of diethyl zinc and an organic ligand. The procedure has been adapted to produce zinc sulfide without the ligand. However, the particle size of the resulting zinc sulfide was not reported.
Optical ceramic precursors having small particles are desirable because they often can be processed under mild conditions into fine-grained ceramics. The resulting fine-grained ceramics are generally low in defects and have improved optical and mechanical properties. Moreover, if the grain size is substantially smaller than the wavelength of light, then light scattering between grains in non-cubic materials is minimized. As a result, a wider range of metal sulfides would be available as optical ceramics.
Metal sulfide powders used as optical ceramic and catalyst precursors must have few impurities. The purity of the final product is dependent on the purity of the starting material. Inorganic starting materials used in conventional syntheses contain impurities which can degrade the performance of the ceramic. Organometallic starting materials of very high purity are easily obtained by distillation, sublimation or recrystallization. However, the purity of the final product may be affected by the presence of residual hydrocarbon in the form of incompletely reacted organometallic material or entrained solvent.